Using Positive DC Offset of Bias RF to Neutralize Charge Build-Up of Etch Features

ABSTRACT

Apparatus, systems and methods for plasma etching substrates are provided. The invention achieves dissipation of charge build-up on a substrate being plasma etched to avoid notching or twisting in high aspect ratio contents and similar features. 
     Charge build-up on a substrate being etched by plasma etching can be dissipated by a method for etching a substrate, the method comprising: providing a plasma processing chamber comprising a chamber enclosure and a substrate support adapted to support a substrate within the chamber enclosure; supporting a substrate on the substrate support; forming a plasma within the chamber enclosure such that a surface of the substrate is in contact with the plasma; etching the substrate by generating a negative bias on the substrate surface relative to the plasma; and intermittently changing the bias on the substrate surface to positive relative to the plasma. The present method can be readily integrated into known plasma processing systems.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. Ser. No. 11/362,409, filed Feb. 23, 2006, now U.S. Pat. No. 7,713,430.

FIELD OF THE INVENTION

This invention relates generally to etching processes used in the semi-conductor industry and, more specifically, to methods and apparatus for controlling charge build-up during plasma etching.

BACKGROUND OF THE INVENTION

Higher performance, lower cost, increased miniaturization of electronic components, and greater density of integrated circuits are ongoing goals of the computer industry. One commonly used technique to increase the density of integrated circuits involves stacking of multiple layers of active and passive components one atop another to allow for multilevel electrical interconnection between devices formed on each of these layers. This multilevel electrical interconnection is generally achieved with a plurality of metal-filled vias (“contacts”) extending through dielectric layers which separate the component layers from one another. These vias are generally formed by etching through each dielectric layer by etching methods known in the industry, such as plasma etching. Plasma etching is also used in the forming of a variety of features for the electronic components of integrated circuits.

As described in U.S. Pat. No. 6,544,895, incorporated herein by reference, in plasma etching, a glow discharge is used to produce reactive species, such as atoms, radicals, and/or ions, from relatively inert gas molecules in a bulk gas, such as a fluorinated gas, such as CF₄, CHF₃, C₂F₆, CH₂F₂, SF₆, or other freons, and mixtures thereof, in combination with a carrier gas, such as Ar, He, Ne, Kr, O₂, or mixtures thereof Essentially, in a plasma etching process comprises: 1) reactive species are generated in a plasma from the bulk gas, 2) the reactive species diffuse to a surface of a material being etched, 3) the reactive species are absorbed on the surface of the material being etched, 4) a chemical reaction occurs which results in the formation of a volatile by-product, 5) the by-product is desorbed from the surface of the material being etched, and 6) the desorbed by-product diffuses into the bulk gas.

In a plasma etching chamber for semiconductor manufacturing, a plasma is maintained by coupling energy from a power source into the plasma, which comprises mobile, positively and negatively charged particles. An electric field, or bias voltage, develops in a sheath layer around the plasma, accelerating the ions toward the semiconductor substrate by electrostatic coupling. Applying an oscillating bias power can modulate the potential difference between the plasma and the semiconductor substrate. The difference between the instantaneous plasma potential and the surface potential defines the sheath potential drop. During the positive voltage phase, the substrate collects electron current from electrons that have enough energy to cross the sheath while during the negative voltage phase, positive ions are accelerated by the sheath voltage drop, strike the substrate, and are collected by the substrate.

However, plasma etching processes (as well as ion implantation and other charge beam processes) may damage the semiconductor substrate and the devices and circuits formed therein or thereon. In particular, electrical charging is a well-known problem that can occur during the plasma processing of semiconductor devices, leading to the degradation of the device performance.

Illustrated in drawing FIG. 6 is the phenomenon of electrical charging on a semiconductor device in the process of a plasma etch. A material layer 244 to be etched is shown layered over a semiconductor substrate 242. A patterned photoresist layer 246 is provided on the material layer 244 for the etching of a via. During the plasma etching process, the patterned photoresist layer 246 and material layer 244 are bombarded with positively charged ions 248 and negatively charged electrons 252 (i.e., the reactive species). This bombardment results in a charge distribution being developed on the patterned photoresist layer 246 and/or the semiconductor substrate 242. This charge distribution is commonly called “feature charging.”

In order for feature charging to occur, the positively charged ions 248 and the negatively charged electrons 252 must become separated from one another. The positively charged ions 248 and negatively charged electrons 252 become separated by virtue of the structure being etched. As the structure (in this example a via 254) is formed by etching, the aspect ratio (height-to-width ratio) becomes greater and greater. During plasma etching, the positively charged ions 248 are accelerated (e.g., as a result of a DC bias at the semiconductor substrate 242) toward the patterned photoresist layer 246 and the material layer 244 in a relatively perpendicular manner, as illustrated by the arrows adjacent positively charged ions 248. The negatively charged electrons 252, however, are less affected by the DC bias at the semiconductor substrate 242 and, thus, move in a more random isotropic manner, as depicted by the arrows adjacent negatively charged electrons 252. This results in an accumulation of a positive charge at a bottom 256 of via 254 because, on average, positively charged ions 248 are more likely to travel vertically toward the substrate 208 than are negatively charged electrons 252. Thus, any structure with a high enough aspect ratio tends to charge more negatively at photoresist layer 246 and an upper portion of the material layer 244 to a distance A (i.e., illustrated with “−” indica) and more positively at the via bottom 256 and the sidewalls 258 of the via 254 proximate the via bottom 256 (i.e., illustrated with “+” indica).

As shown in drawing FIG. 7, the positively charged via bottom 256 deflects the positively charged ions 248 away from the via bottom 256 and toward the sidewalls 258 of the via 254, as a result of charge repulsion. The deflection results in an etching of the sidewalls 258 proximate the via bottom 256, known as “notching” or “twisting.” Furthermore, the presence of the positively charged via bottom 256 slows the positively charged ions 248 as they approach the positively charged via bottom 256, thereby reducing etching efficiency.

Current High Aspect Ratio Contacts (HARCs) are known to twist at aspect ratios greater than about 20:1. Twisting is the deviation of the bottom of the contact from the center line of the etch front. Twisting is caused by asymmetric charge build-up in and around the contact causing a lateral deviation of the ion projectory. The twisting may be so serious that the etch processes actually generate corkscrew shaped contacts. The twisting of the contact is a concern for shorting contacts to other structures or to each other. Certain tools and chemistries help reduce the twisting, but all tools show this phenomenon.

Twisting is a current failure mechanism for contacts on 95 nm parts and has required extensive process development to overcome. HARC etches are probably the most difficult etch needed on DRAM parts and have very tight constraints on profiles, film selectivities, and Critical Dimensions (CDs). The twisting behavior of contacts will limit the aspect ratio of contacts that can be etched in the near future and there is little research to understand this phenomena and no known solution to eliminate it.

In a standard plasma etch system the ion angular distribution is very anisotropic whereas the electron angular distribution is very isotropic. For HARC features the electrons will mainly strike the contact near the top of the feature; while ions will reach the bottom of the feature. This is what causes the top of the contacts to charge negative while the bottom of the contacts charge positive. Small asymmetries in the top of the contacts due to photolithography or polymer loading will cause asymmetric charging at the top of the contact leading to bending of the incident ions. This will then cause the contact to etch faster on one side of the contact due to increased ion flux to this area.

SUMMARY OF THE INVENTION

A hallmark of the method of this invention is the use of DC power to reverse the sheath voltage in a plasma etch process, thereby dissipating charge build-up on the substrate being etched.

In one aspect, the invention provides a plasma etching apparatus. In one embodiment, the apparatus comprises a plasma processing chamber with a chamber enclosure; a support adapted to hold or support a substrate within the chamber enclosure; a radio frequency (RF) power source configured to impose a negative bias across a substrate supported by the substrate support; a DC power supply configured to generate a positive voltage with respect to the plasma within the chamber and in intermittent electrical communication with the substrate support; and a control switch to select between connected and unconnected electrical communication between the DC power supply and the substrate support.

In another embodiment of the etching apparatus, the DC power supply comprises a function generator power supply configured to generate a varying positive DC voltage with respect to plasma within the chamber enclosure, wherein the function generator power supply is in electrical communication with the substrate support.

In yet another embodiment of the etching apparatus, the DC power supply comprises a function generator power source configured to generate a variable voltage signal, and the apparatus further includes a signal amplifier wherein the alternating voltage signal imposes a bias relative to the plasma across the surface of a substrate supported by the substrate support.

In another aspect, the invention provides methods for etching a substrate. In one embodiment, the method comprises forming a plasma within a plasma processing chamber containing a substrate to be etched, and etching the substrate by generating a negative bias on the surface of the substrate relative to the plasma; and intermittently changing the bias on the surface of the substrate to positive relative to the plasma. Exemplary substrates that can be etched by the methods of the invention include insulating materials (e.g., oxides, nitrides, polymers) utilizing a plasma comprising ionizable gases.

In another embodiment, the method comprises applying an RF signal to generate a negative bias on the surface of the substrate relative to the plasma to cause ions in the plasma to etch the substrate, and applying a varying DC voltage to the substrate support to intermittently change the bias on the surface of the substrate to positive relative to the plasma.

In a further embodiment, the method comprises applying a varying RF signal to generate a bias on the surface of the substrate, the bias varying from negative to positive relative to the plasma such that, when the bias is negative, ions in the plasma etch the substrate.

In yet another embodiment, the method comprises applying an RF signal to generate a negative bias on the surface of the substrate relative to the plasma to cause ions in the plasma to etch the substrate, generating a constant DC voltage signal that is positive relative to the plasma, and selectively operating a control switch between an open position that precludes electrical communication between the constant DC voltage signal and the substrate support and a closed position that establishes electrical communication between the constant DC voltage signal and the substrate support, such that when the switch is in a closed position, the constant DC voltage signal changes the bias of the surface of the substrate to positive relative to the plasma.

The invention provides devices and methods to etch high aspect ratio features. Features such as vias and contact openings that etched according to the invention advantageously are substantially free of feature charging, notching and/or twisting. The present invention can be readily integrated into conventional plasma processing systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below with reference to the following accompanying drawings, which are for illustrative purposes only. Throughout the following views, reference numerals will be used in the drawings, and the same reference numerals will be used throughout the several views and in the description to indicate same or like parts or steps.

FIG. 1 is a flow diagram illustrating steps in etching a substrate according to an embodiment of a method of the present invention.

FIG. 2 is a schematic diagram showing one embodiment of an apparatus of the invention.

FIG. 3 is a schematic diagram showing another embodiment of an apparatus of the invention.

FIG. 4 is the schematic diagram of another embodiment of an apparatus of the invention.

FIGS. 5A and 5B are cross-sectional views of a contact being etched by the process of FIG. 1.

FIG. 6 is a cross-sectional view of a via during a prior art etching process which results in the phenomenon of feature charging.

FIG. 7 is a cross-sectional view of a via during a prior art etching process wherein feature charging results in the deflection of positively charged ions away from the bottom of the via and toward the sidewalls of the via.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, references are made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. Although the examples presented are directed to the formation of an opening or via, it is understood that the present invention may be utilized in a variety of feature formation and plasma processes.

In the context of the current application, the term “semiconductor substrate” or “semiconductive substrate” or “semiconductive wafer fragment” or “wafer fragment” or “wafer” will be understood to mean any construction comprising semiconductor material, including but not limited to bulk semiconductive materials such as a semiconductor wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure including, but not limited to, the semiconductive substrates, wafer fragments or wafers described above. Furthermore, when references made to a wafer or substrate in the following description, previous process steps may have been used to form regions or junctions in the base semiconductor structure or foundation.

FIG. 1 shows a flowchart of an embodiment of an etching process 101 according to the invention for plasma etching a substrate while FIGS. 5A and 5B, which will be discussed concurrently with FIG. 1, show schematic representations of a substrate being etched by plasma in accordance with this invention. FIGS. 5A-5B illustrate a wafer fragment 340 with a substrate 344 to be etched overlying material layer 342, which can comprise an unprocessed semiconductor wafer or other substrate, the wafer with various process layers formed thereon including one or more semiconductor layers or other formations, and active or operable portions of semiconductor devices. Exemplary materials for substrate 344 include oxides, nitrides, polymers, silicon, and other insulating and semiconducting materials.

In a first Step 103, wafer 340 with substrate 344 to be etched bearing a patterned mask layer 346 (e.g., photoresist over an oxide layer) is loaded into a plasma processing chamber onto a support. Etch gases are fed into the process chamber, and a plasma is then generated within the chamber in Step 105.

The plasma generation is decoupled from wafer bias. Decoupling can be carried out with a variety of plasma reactors. Examples of sources used to create a plasma include an electron cyclotron resonance (ECR) source, an inductively coupled plasma (ICP), capacitive discharges, microwave discharges (e.g., applied by a surface wave process, etc.), a helicon wave source, etc., all of which are well known in the art. The plasma may be generated utilizing a conventional etching device, such as an etcher available from Applied Materials under the trade designation of Enabler etcher, a Kiyo TCP Oxide Etcher available from Lam Research Corporation, or other plasma etcher. It should be readily apparent to one skilled in the art that, depending upon the particular etching apparatus utilized to generate the plasma, various parameters provided herein can be modified to accomplishing the objectives of the invention.

By way of example and not limitation, an inductively coupled plasma (ICP) source of a dry or plasma processing apparatus, such as a Enabler etcher (Applied Materials Corporation, San Jose, Calif.) with a top and a bottom electrode (dual power chamber), can be used for the inventive plasma etching process. In the Enabler etcher, the top and bottom electrodes are not coupled. The top electrode is configured to provide a plasma source (“top power”), and the bottom electrode is configured to provide a bias voltage (“bottom power”) and a wafer chuck. In the Enabler etcher, the bias power is provided as a RF signal to the bottom electrode. By increasing bottom RF power the bias voltage is increased across the substrate positioned on the bottom electrode.

In Step 107, an RF signal is supplied to the chamber. Self-rectification of the RF signal produces a negative bias on the surface of the wafer 340 with respect to the plasma. The RF power, and resultant negative substrate bias, provides the energy to drive the plasma ions to the surface of the wafer 340. Thereby, in Step 109, the plasma ions etch the portion of the substrate 344 (e.g., oxide layer) exposed through openings of the patterned mask 346 to form feature 354 (e.g., a via or contact). As shown in FIG. 5A, positive ions 348 will travel anisotropically toward the wafer 340 while negative ions 352 and electrons 353 travel more isotropically. The result is the typical charge separation with the photoresist layer 346 and the upper portion of the substrate 344 being more negatively charged and the bottom 356 of the etched feature 354 being more positively charged.

In Step 111, the power is supplied to the wafer 340 (normally through the substrate support pedestal (not shown)) to intermittently reverse the sheath voltage from negative to positive. The reversal of the sheath voltage allows charge build-ups on the upper portion of the substrate 344 and at the bottom 356 of the etched feature 354 to dissipate. As shown in FIG. 5B, when the sheath voltage is positive, negative ions 352 and electrons 353 travel anisotropically toward the wafer 340, which facilitates negatively charged particles reaching the bottom 356 of feature 354 to offset the positive charge buildup from Step 109. Likewise, the positive ions 348 travel more isotropically when the sheath voltage is positive, which results in the positive ions 348 striking mainly the surface of the wafer 340 and upper portions of the etched feature 354 to offset the negative charge buildup at these portions of the wafer.

The intermittent reversal of the substrate bias can be accomplished by alternatively switching back and forth from a constant DC power supply, by providing a pulsed signal using a function generator and signal amplifier, or by using a pulsed DC power supply. These methods of reversing the bias and an apparatus for accomplishing the result are discussed in detail below.

In Step 113, if further etching is required, Steps 107, 109, 111 are repeated as necessary. If no further etching is required, the process can be terminated.

An embodiment of an apparatus for carrying out the method of invention is illustrated in FIG. 2. As shown, the plasma processing chamber 201 includes a chamber enclosure 203. Enclosure 203 is electrically connected to ground 209. A plasma source 205 generates plasma 207 within chamber enclosure 203. A wafer chuck 211 supports a substrate 213 (e.g., wafer) within chamber enclosure 203. Typically, wafer chuck 211 is an electrostatic chuck (ESC). Wafer 213 has a surface 215 exposed to the plasma for etching.

Attached to the chuck 211 is a function generator 217. The purpose of function generator 217 is to generate an alternating electric signal whereby the voltage changes over time, which can be accomplished with conventional apparatus and processes. For this embodiment, the function generator provides pulses of positive voltage with respect to ground, which alternate with pulses of less positive voltage (including 0V and negative voltage if desired). The shape, frequency, amplitude and duration of such pulses can be adjusted depending on the etching performance.

A function generator is a device that can produce various waveform patterns of voltage at a variety of frequencies and amplitudes. Function generators are well-known in the art and are frequently used as test equipment. Most function generators allow the user to choose the shape of the output from a number of options. Typical waveforms include: a square wave where the signal goes directly from high to low voltage; a sine wave where the signal curves like a sinusoid from high to low voltage; and, a triangle wave where the signal goes from high to low voltage at a fixed rate. These waveforms can be either repetitive, or single-shot (once only) in which case some kind of triggering source is required (internal or external).

The amplitude control on a function generator varies the voltage difference between the high and low voltage of the output signal. The direct current (DC) offset control on a function generator varies the average voltage of a signal relative to the ground. The frequency control of a function generator controls the rate at which output signal oscillates. On some function generators, the frequency control is a combination of different controls. One set of controls chooses the broad frequency range (order of magnitude) and the other selects the precise frequency. This allows the function generator to handle the enormous variation in frequency scale needed for signals.

Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively “sweep” the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit.

The signal from function generator 217 is then amplified by amplifier 221 and fed into the wafer chuck 211. An RF filter 223 may be used between the amplifier 221 and the wafer chuck 211 to prevent feedback communication of the RF signal to the function generator. The net result is that the sheath voltage is forced to predetermined negative values. The change in sheath voltage accelerates negatively charged species (e.g., electrons and negative ions) to the surface of the wafer and dissipates the positive charge build-up at the bottom of the contacts, as depicted in FIGS. 5A-5B discussed earlier.

FIG. 3 illustrates another embodiment of a plasma etch apparatus according to the invention. The parts and functions of labeled components of FIG. 3 are identical to the same numbered components as in FIG. 2. The process of an apparatus of FIG. 3 differs in that an RF Power is supplied to the wafer 213 in a conventional manner, as depicted by the dashed box 225. The RF power supply 227 is connected to a ground 229. The signal from RF power supply 227 is fed to chuck 211 through match box 231. The net result is that the sheath voltage is forced to predetermined negative values for a short period of time, thereby accelerating negatively charged species to the surface of the wafer and dissipating the positive charge build-up at the bottom of the contacts (FIGS. 5A-5B).

It is common to use tuning components to help match the impedance of the plasma to the output impedance of the RF power supply. These components, usually a shunt capacitor to ground and a series 3-4-turn inductor, are located along with the series capacitor in the “matchbox,” which is physically located adjacent to the cathode position. The inductor is fixed, and both of the capacitors (shunt and series) are variable. A control circuit within the matchbox controller senses the reflected power (from the matchbox and plasma back to the power supply) and adjusts the variable capacitors to minimize the reflected power. Usually this is done automatically by means of reversible motor drives on the capacitors, but occasionally laboratory-based systems will have manual controls for the tuning network.

FIG. 4 illustrates another embodiment of a process and a plasma etch apparatus according to the invention. Plasma processing chamber 201, chamber enclosure 203, plasma source 205, plasma 207, ground 209, wafer chuck 211, substrate 213, and surface 215 are as described with respect to the apparatus of FIG. 2. The RF power supply shown in dashed box 225 is as described with respect to the apparatus of FIG. 3. A DC power supply 233 that generates a constant positive voltage with respect to the plasma 207 is additionally attached to the wafer chuck 211. Power supply 233 is connected to ground 235. The positive voltage signal from DC power supply 233 is fed through switch 237 that can alternate back and forth between open (unconnected) and closed (connected) positions. When connected, the DC power supply changes the sheath voltage to determined positive values for the duration of the connection. Opening the connection allows the negative bias to reestablish on wafer surface 215.

In the present example of plasma etching an oxide layer, a preferred source for plasma 207 is a fluorocarbon or hydrofluorocarbon feed gas. When utilized to generate a plasma, those feed gases dissociate resulting in fragments for use in an oxide etching process, for example, C_(x)H₃F₂ ⁺ ions or C_(x)F₂ ⁺ ions. Exemplary fluorocarbon or hydrofluorocarbon gases include CF₄, CHF₃, C₂F₆, C₃F₆, CH₂F₂, C₂HF₅, among others, and can be used alone or in combination with another gas such as hydrogen (H₂) or oxygen (O₂) to adjust the nature of the carbon- and fluorine-containing ions within the plasma.

Thus, the present invention is capable of providing a simple and controllable method of affecting the quality and efficiency of plasma etching and is easily implemented on most existing plasma reactors. The present invention is useful in etching apertures having a length-to-diameter ratio of 20:1 or greater in insulating materials deposited by chemical vapor deposition techniques. Such insulating materials include oxides, nitrides, polymers, combinations thereof, etc. Furthermore, although the examples presented are directed to the formation of an opening or via, it is understood that the present invention may be utilized in a variety of feature formation and plasma processes.

The reduction of charge build-up could also be applied to line and space structures or any other feature where the build-up of charge is causing negative effects. Other negative effects of charge build-up (besides twisting) are notching, aspect ratio dependent etch rate, profile distortion of the etched feature (bow and tapered profiles for example), and etch stop to name a few. One application that is significantly different would be the reduction of charge build-up that leads to micro-arcing during the etch process. Micro-arcing can occur at the edge of the wafer where the lithography pattern is disturbed while forming discontinuous films. We believe this may be caused by charge build-up of the feature that then discharge when another conducting layer is contacted. This causes a micro-arc and damages the wafer. Micro-arcs can also cause a chain reaction leading to larger arcing events damaging the wafer and the chamber.

In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents. 

1. A method for etching a substrate, comprising: generating a plasma within a processing chamber to contact a surface of a substrate situated on a support within the chamber; etching the substrate to form an opening by providing an RF voltage signal to the processing chamber to impose a negative bias on the surface of the substrate relative to the plasma; generating a constant DC voltage signal that is positive relative to the plasma at an amplitude, frequency and duration effective to cause the negative bias on the substrate surface imposed by the RF voltage signal to change to a positive bias on the substrate surface relative to the plasma; and selectively operating a control to switch between an open position that precludes electrical communication between the constant DC voltage signal and the substrate support and a closed position that establishes electrical communication between the constant DC voltage signal and the substrate support to intermittently supply the constant DC voltage signal to the substrate support to cause the negative bias on the substrate surface imposed by the RF voltage signal to change to a positive bias on the substrate surface relative to the plasma.
 2. The method of claim 1, wherein selectively operating the control delivers a pulsed positive DC voltage signal ranging from a positive voltage to a less positive voltage including a zero voltage.
 3. The method of claim 1, wherein the constant DC signal is a waveform pattern signal.
 4. The method of claim 3, wherein the waveform pattern is a sinusoid waveform pattern.
 5. The method of claim 3, wherein the waveform pattern is a triangle waveform pattern.
 6. The method of claim 1, wherein selectively operating the control supplies the constant DC voltage signal to the substrate support to decrease charge build-up within the opening.
 7. The method of claim 1, wherein plasma generation is decoupled from substrate bias.
 8. The method of claim 1, wherein the plasma is generated by a plasma source selected from the group consisting of electron cyclotron resonance source, an inductively coupled plasma source, a surface wave plasma source, a helicon wave plasma source, and a capacitive coupled plasma source.
 9. A method for etching a substrate, comprising: etching a surface of substrate to form an opening by providing an RF voltage signal in the presence of a plasma to impose a negative bias on the surface of the substrate relative to the plasma; and selectively operating a control to intermittently supply a constant positive DC voltage signal to the substrate support at an amplitude, frequency and duration effective to cause the negative bias on the substrate surface imposed by the RF voltage signal to change to a positive bias on the substrate surface relative to the plasma, wherein the control is switched between an open position that prevents and a closed position that permits electrical communication between the constant DC voltage signal and the substrate support.
 10. A method for etching a substrate, comprising: forming a plasma within a processing chamber to contact a surface of a substrate situated within the chamber; etching the substrate to form an opening by providing an RF voltage signal to the processing chamber to generate and impose a negative bias on the substrate surface relative to the plasma, the bias varying from negative to positive relative to the plasma, wherein ions in the plasma etch the substrate when the bias is negative; and selectively operating a control to intermittently deliver a positive DC voltage signal as a pulsed signal ranging from a positive voltage to a less positive voltage including a zero voltage at an amplitude, frequency and duration effective to cause the negative bias on the substrate surface imposed by the RF voltage signal to change to a positive bias on the substrate surface relative to the plasma.
 11. An etching apparatus, comprising: a plasma processing chamber comprising a chamber enclosure and a substrate support within the chamber enclosure; a plasma generator coupled to the processing chamber; an RF voltage power source configured to generate and impose a negative bias across a surface of a substrate supported by the substrate support relative to a plasma to etch an opening in the substrate; a DC power supply in selectable electrical communication with the substrate support, the DC power supply configured to generate a constant positive DC voltage signal to the substrate support when the DC power supply is in electrical communication with the substrate support to cause the negative bias on the substrate surface imposed by the RF voltage power source to change to a positive bias on the substrate surface relative to the plasma; and a control to switch between an open position that precludes electrical communication between the constant DC voltage signal and the substrate support and a closed position that establishes electrical communication between the constant DC voltage signal and the substrate support.
 12. The apparatus of claim 11, wherein plasma generation is decoupled from substrate bias.
 13. The apparatus of claim 11, wherein the plasma generator is configured for generating a plasma source selected from the group consisting of electron cyclotron resonance source, an inductively coupled plasma source, a surface wave plasma source, a helicon wave plasma source, and a capacitive coupled plasma source.
 14. An etching apparatus, comprising: a plasma processing chamber comprising a chamber enclosure and a substrate support within the chamber enclosure; a plasma generator coupled to the processing chamber; an RF voltage power source configured to generate and impose a negative bias across a surface of a substrate supported by the substrate support relative to a plasma to etch an opening in the substrate; a DC power supply in selectable electrical communication with the substrate support, the DC power supply configured to generate a pulsed positive DC voltage signal to the substrate support when the DC power supply is in electrical communication with the substrate support, the pulsed DC voltage signal ranging from a positive voltage to a less positive voltage including a zero voltage at an amplitude, frequency and duration to intermittently cause the negative bias on the substrate surface imposed by the RF voltage power source to change to a positive bias on the substrate surface relative to the plasma; and a control operable to preclude and establish electrical communication between the DC voltage signal and the substrate support.
 15. An etching apparatus, comprising: a plasma processing chamber comprising a chamber enclosure and a substrate support within the chamber enclosure; a plasma generator coupled to the processing chamber; an RF voltage power source configured to generate and impose a negative bias across a surface of a substrate supported by the substrate support relative to a plasma to etch an opening in the substrate; and a function generator power supply in electrical communication with the support substrate and configured to generate a pulsed positive DC voltage signal to the substrate support ranging from a positive voltage to a less positive voltage including a zero voltage at an amplitude, frequency and duration to intermittently cause the negative bias on the substrate surface imposed by the RF voltage power source to change to a positive bias on the substrate surface relative to the plasma.
 16. The apparatus of claim 15, wherein the function generator is configured to pulse the positive DC signal in a waveform pattern signal.
 17. The apparatus of claim 16, wherein the waveform pattern is a sinusoid waveform pattern.
 18. The apparatus of claim 16, wherein the waveform pattern is a triangle waveform pattern.
 19. An etching apparatus, comprising: a plasma processing chamber comprising a chamber enclosure and a substrate support within the chamber enclosure; a plasma generator coupled to the processing chamber; a function generator power supply in electrical communication with the support substrate and configured to generate a variable voltage signal; and a filter operable to convert the variable voltage signal to an RF signal, wherein the RF signal imposes a bias, relative to the plasma, across a substrate supported by the substrate support. 